The present invention relates to an actuator which can convey a workpiece, etc., for example, under the displacing action of the moving member and the controlling device therefor.
The actuator has been conventionally used for such a purpose as conveying a workpiece. As shown in FIG. 18, this actuator 200 comprises an elongate guide base 201 having a rectangular-shaped cross section; a feed screw shaft 202 both ends whereof are supported by bearing blocks (not shown) so that it can be freely rotated, and whereto the rotation drive force of the drive source (not shown) is transmitted; and a moving member 203 wherein a threaded hole engaging the feed screw shaft 202 is formed as a through-hole, and which is moved in both directions along the longitudinal axis of the guide base 201 under the rotating action of the feed screw shaft 202.
By providing balls 204 between the guide base 201 and the moving member 203, this actuator 200 is configured so that the moving member 203 can be smoothly moved in both directions under the rolling action of the balls 204.
However, with the actuator 200 according to the prior art, when, for example, the thread of the feed screw shaft 202 is worn out, resulting in the linear accuracy for the moving member 203 being lowered, or the feed screw shaft 202 is to be replaced with other feed screw shaft having a different diameter, or other driving force transmission member such as a ball screw shaft, the feed screw shaft 202, moving member 203, etc., which form a unit, must be integrally replaced, which makes the replacement operation cumbersome. In addition, not only the feed screw shaft 202 but also the moving member 203 must be replaced, and thus a problem of the cost being increased arises.
Also, with the actuator 200 according to the prior art, when the user intends to use the actuator, meeting specifications different from the normal ones to suit to the operating environment, it is difficult to reassemble the actuator 200 to the desired configuration after disassembling, and the reassembling operation to provide the desired configuration is extremely cumbersome, and thus a problem of the cost rising is caused.
Conventionally, a motor-operated actuator 300 as shown in FIG. 19, for example, is known. The motor-operated actuator 300 as shown in FIG. 19 is formed to be elongate along the longitudinal axis, comprising a frame 320 provided with a set of substantially parallel guide rails 318a and 318b, and a moving member 330 which is displaced along the longitudinal axis of the frame 320 under the guide by the guide rails 318a and 318b. 
In a concavity 321 defined in the frame 320, a ball screw shaft 316 is supported along the longitudinal axis, and to a moving member 330 is fixed a bearing block 341 wherein a threaded hole (not shown) engaging the ball screw shaft 316 is formed. In addition, the motor-operated actuator 300 comprises a supporting block 342 which is fixed to one end portion of the frame 320 and supports the ball screw shaft 316 so that it can be freely rotated, and a brushless motor 322 fixed to the other end of the frame 320 and constituting the driving section for rotation-driving the ball screw shaft 316.
With the motor-operated actuator 300 configured as above, when the brushless motor 322 is driven by supplying the power to the brushless motor 322, the rotation drive force of the rotor (not shown) of the brushless motor 322 is transmitted to the ball screw shaft 316, resulting in the ball screw shaft 316 being rotation-driven in the set direction.
As the ball screw shaft 316 is rotation-driven, the moving member 330 is smoothly and linearly displaced along the guide rails 318a and 318b through the bearing block 341 engaged with the ball screw shaft 316, and thus the workpiece (not shown) loaded on the moving member 330 can be conveyed.
The numerals 310a and 310b denote stoppers fixed to one end and the other end of the frame 320, respectively, limiting the movable range for the moving member 330.
However, for a conventional motor-operated actuator 300 as described above, it has been desired that, when the workpiece loaded on the moving member 330 is conveyed along the frame 320, the torque for the drive source be limited for such a purpose as slowing down the movement of the moving member 330 in such a case as that when a foreign matter or other is sandwiched between the moving member 330 and the stopper 310a or 310b at either end of the frame 320, or when the moving member 330 is struck against the stopper 310a or 310b at either end of the frame 320.
This is also true when the motor-operated actuator 300 is installed vertically so that the moving member 330 is displaced in a vertical direction, and when the moving member 330 is displaced downward, the lowering speed is increased by the rotation drive force of the drive source and the weight of the workpiece itself, thus it has been required to allow no foreign matter or other to be sandwiched between the moving member 330 and the stopper 310a or 310b. 
Thus, as a controlling device for a conventional motor-operated actuator 300, a friction plate (not shown) or other is provided between the drive source and the ball screw shaft 316 so as to serve as a torque limiter, and when an excessive load is impressed, the friction plate is caused to slip so that an excessive drive force is prevented from being applied to the moving member 330, thus torque control of the drive source being performed.
However, with the conventional method as stated above, the tightening force for the torque limiter must be preset at a desired value before the drive source is assembled, and in such a case as that when the weight of the workpiece itself is to be changed due to the change of the workpiece to be loaded, it is difficult to change the torque limit value for the torque limiter, thus a problem of the degree of freedom of changing the torque limit value being absent has been presented.
The present invention is intended to offer an actuator and a controlling device therefor which allows only the driving force transmission shaft to be replaced conveniently and inexpensively, and reassembled conveniently to suit to the operating environment, yet allows the torque limit value for the rotation drive source to be easily set and changed.
With the actuator according to the present invention, an opening which allows the driving force transmission shaft to be inserted from the direction perpendicular to the axis thereof is formed in the moving block, and thus assembling and maintenance operations for the driving force transmission shaft can be made with convenience.
Further, with the controlling device for the actuator according to the present invention, the level of the manipulated variable control signal which controls the rotational speed of the rotation drive source to the speed based on the generated speed pattern is level-limited to the level of the torque limit value signal, and on the basis of the level-limited manipulated variable control signal, the amount of current supplied to the rotation drive source for the actuator is controlled, which results in the torque for the rotation drive source being controlled to the level of the torque limit limit value signal.
Therefore, the torque for the rotation drive source is controlled on the basis of the torque limit value signal, and the torque limit value signal can be easily set, thus, the torque setting for the rotation drive source can be easily changed to suit to the operating conditions, which allows the torque to be set at an optimum value for the workpiece conditions.